The present invention relates to an instrument for assaying radioactivity. More particularly, the invention relates to a photon detector having a solid scintillator and a single photomultiplier tube.
The counting of nuclear disintegrations emitted from select specimens is frequently accomplished with scintillator materials, photomultiplier tubes and associated circuitry including a pulse height analyzer. For a typical system a specimen of radioactive material is admixed with a liquid scintillator or is placed in the vicinity of a solid scintillator in close proximity to a photomultiplier tube. As the radioactive material decays, the emitted particles interact with the scintillator producing light photons which are collected by the photomultiplier tube. The tube in turn provides an electric signal output and with suitable intermediate circuitry the pulse height analyzer provides an output count for each photomultiplier tube signal which meets some preselected amplitude requirements. A practical consideration which the described system has to contend with is the appropriate recognition of the output signals from the photomultiplier tube. These tubes have a high gain and often produce spurious signals which are simply noise. This noise is particularly troublesome to any attempt to quantitatively analyze a radioactive sample particularly under circumstances in which the count rate from the sample is relatively low or the energy of the radiation from the active nuclide is low. These considerations are fundamental concerns in the design of nuclear detection instrumentation.
One of the more common techniques for distinguishing the events of interest from noise in the output signals from the photomultiplier tube is to use two photomultiplier tubes to collect photons from the scintillation material and compare their output signals. In order for a signal to be considered by the pulse height analyzer a coincident signal must be received from each of the tubes thereby essentially eliminating the possibility of random noise being confused with a genuine nuclear event in the scintillator. Detection instruments with two photomultiplier tubes are routinely used on a commercial basis although there is incentive to continually upgrade such systems particularly if they can be made less elaborate or more efficient as a counter for particles of certain energies.
One attempt at simplifying a solid scintillator system is described by D. Landis et al in a paper entitled "The Application of Pulse Shape Discrimination to Separating Phototube Noise Pulses from Scintillation Pulses" dated Sept. 1, 1964. The device is simple in that only one photomultiplier tube is required. The essence of its operation involves measuring the width of the filtered pulse from the photomultiplier tube by timing the interval between the leading edge and zero crossing. Those pulses shorter than a preselected interval are attributed to noise and are therefore discarded while those pulses exceeding a minimum time interval are considered valid pulses resulting from a nuclear disintegration captured by the scintillator and are recorded. A similar pulse discrimination scheme is used with a flow gas system disclosed by Colmenares et al in a 1974 article entitled "A European Doped Calcium Fluoride Scintillator System for Low Level Tritium Detection." While the Landis et al and Colmenares et al type systems are useful, each has drawbacks. One such drawback, for example, is a poor signal to noise ratio for low energy pulses.
In an article by L. N. Mackey et al entitled "High efficiency Solid Scintillation Radioactivity Detection for High Performance Liquid Chromatography", the authors disclosed related technology applied to the design of a carbon-fourteen monitor. The system uses optimized electronics and particularly sized solid scintillator material in what is referred to as a heterogenous detector, i.e., the radioactive effluent is recoverable intact since it does not become mixed with a liquid scintillator. The system apparently is very good although two photomultiplier tubes are required.